1Introduction to the Oracle Database

This book contains information relating to both Oracle Database Standard Edition and Oracle Database Enterprise Edition. Some of the features and options documented in this chapter are available only if you have purchased the Oracle Database Enterprise Edition. See Oracle Database New Features for information about the differences between Oracle Database Standard Edition and Oracle Database Enterprise Edition.

Oracle Database Architecture

An Oracle database is a collection of data treated as a unit. The purpose of a database is to store and retrieve related information. A database server is the key to solving the problems of information management. In general, a server reliably manages a large amount of data in a multiuser environment so that many users can concurrently access the same data. All this is accomplished while delivering high performance. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.

Oracle Database is the first database designed for enterprise grid computing, the most flexible and cost effective way to manage information and applications. Enterprise grid computing creates large pools of industry-standard, modular storage and servers. With this architecture, each new system can be rapidly provisioned from the pool of components. There is no need for peak workloads, because capacity can be easily added or reallocated from the resource pools as needed.

The database has logical structures and physical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.

For example, you could run different applications on a grid of several linked database servers. When reports are due at the end of the month, the database administrator could automatically provision more servers to that application to handle the increased demand.

Grid computing uses sophisticated workload management that makes it possible for applications to share resources across many servers. Data processing capacity can be added or removed on demand, and resources within a location can be dynamically provisioned. Web services can quickly integrate applications to create new business processes.

Grid computing offers high performance and scalability, because all computing resources can be flexibly allocated to applications as needed.

Oracle Database enables enterprise grid computing in the following ways:

Performance and scalability with low cost hardware clusters, like Itanium and Linux.

Reliability: Continuous availability of data and applications

Security and privacy: security features that lets you share enterprise grid resources with confidence that privacy is maintained

Self-management: Oracle infrastructure automates many functions so that a single administrator can manage hundreds of servers.

Distributed computing: Oracle has advanced integration features that allow applications and data to run anywhere in the network.

Difference between a cluster and a grid: Clustering is one technology used to create a grid infrastructure. Simple clusters have static resources for specific applications by specific owners. Grids, which can consist of multiple clusters, are dynamic resource pools shareable among many different applications and users. A grid does not assume that all servers in the grid are running the same set of applications. Applications can be scheduled and migrated across servers in the grid. Grids share resources from and among independent system owners.

At the highest level, the idea of grid computing is computing as a utility. In other words, you should not care where your data resides, or what computer processes your request. You should be able to request information or computation and have it delivered - as much as you want, and whenever you want. This is analogous to the way electric utilities work, in that you don't know where the generator is, or how the electric grid is wired, you just ask for electricity, and you get it. The goal is to make computing a utility, a commodity, and ubiquitous. Hence the name, The Grid. This view of utility computing is, of course, a "client side" view.

From the "server side", or behind the scenes, the grid is about resource allocation, information sharing, and high availability. Resource allocation ensures that all those that need or request resources are getting what they need, that resources are not standing idle while requests are going unserviced. Information sharing makes sure that the information users and applications need is available where and when it is needed. High availability features guarantee all the data and computation is always there, just like a utility company always provides electric power.

Oracle provides substantial grid computing technology, available today, that can help you capitalize on the grid. Oracle is the best on commodity clusters, a hardware platform that many believe would come to dominate the grid. Oracle possesses the key technology differentiators -- Oracle Real Application Clusters, Oracle Streams, Oracle Transportable Tablespaces -- for building the grid. Only Oracle delivers the operational characteristics -- portability, RAS, security, and scalability -- necessary for the grid.

Recently, ideas behind grid computing have evolved, due in large part to advances in hardware and networking technologies and the drop in costs associated with these technologies. New high-volume processors and inexpensive blade servers, for example, are extremely affordable compared to their high-end SMP predecessors. Instead of scavenging resources as in the older model of grid computing, a business could relatively cheaply build a blade server farm whose resources could be dynamically and automatically allocated to the areas of the enterprise that required the computing power.

In addition to operating on low-cost, off-the-shelf hardware that can be quickly assembled to create a relatively large-scale operation, this vision of grid computing allows for a high level of flexibility in meeting existing and future computing needs.

Oracle Technologies that Enable the Grid

Oracle has been working for years on technologies that support and enable grid computing.

Computing resource provisioning is one of the most important capabilities of a grid. This enables computing resources to be dynamically provisioned to applications as required. Resources must be appropriately allocated based on business priorities and demand. Oracle provides a number features for computing resource provisioning, including:

Real Application Clusters. RAC is a cluster database with a shared cache architecture that runs on multiple machines, attached through a cluster interconnect and a shared storage subsystem. An Oracle RAC database not only appears like a single standard Oracle Database to users, but the same maintenance tools and practices used for a single Oracle Database can be used on the entire cluster. All standard backup and recovery operations, including the use of Recovery Manager, work transparently with RAC. All SQL operations, including data definition language and integrity constraints, are also identical for both configurations. The most important part of RAC, however, is the ability to manage your workload—to add nodes or relinquish nodes on demand—based on your business processing needs.

Automatic Storage Management. Oracle recommends using Automatic Storage Management (ASM) for your database files and a cluster file system for the Oracle home. ASM simplifies the administration of Oracle database files. Instead of managing many database files, ASM requires you to manage only a small number of disk groups. You can define a particular disk group as the default disk group for a database.

Oracle Scheduler. Oracle Scheduler provides many capabilities to schedule and perform business and IT tasks, called jobs, in a grid.

Information provisioning means delivering information to users whenever they need it, regardless of where it resides on the grid. To process information on any available resource, the grid must efficiently share information across distributed systems. The grid must also provide access to data residing on heterogeneous systems—database systems from multiple vendors and file systems. Oracle provides a broad set of features and tools for information provisioning on a grid, including the following:

Oracle Transportable Tablespaces. Transportable Tablespaces allows Oracle datafiles to be unplugged from a database, moved, or copied to another location, and then plugged into another database. Unplugging or plugging a datafile involves reading or loading only a small amount of metadata. Transportable Tablespaces also supports simultaneous mounting of read-only tablespaces by two or more databases.

Oracle Streams. Some data needs to be shared as it is created or changed, rather than occasionally shared in bulk. Oracle Streams can stream data between databases, nodes, or blade farms in a grid and can keep two or more copies in sync as updates are applied. It also provides a unified framework for information sharing, combining message queuing, replication, events, data warehouse loading, notifications, and publish/subscribe into a single technology.

A combination Streams/Transportable Tablespace feature enables a self-propelled database. With a single command, you can take a tablespace from one database, ship the tablespace to another database, reformat the tablespace if the second database is on a different operating system, plug this tablespace into the second database, and start syncing the tablespace with the changes happening in the first database. If the second database is on a grid, then you have just migrated your application to a grid with a single command.

Easy OCI/JDBC Install. With Oracle Database, any application using Oracle Call Interface (OCI) can easily install a small footprint version of the Oracle client files without installing and configuring the entire client. These applications include Java database connectivity (JDBC) type-2 driver applications. OCI and the JDBC type-2 driver (which requires OCI) can easily be installed by downloading a small subset of the Oracle client files and updating a few environment variables to point to the location of the downloaded libraries. This means your grid client doesn't need to install any Oracle software and yet you get easy, secure access to data from an Oracle database running on a grid.

Distributed SQL and Distributed Transactions. Oracle Distributed SQL allows grid users to efficiently access and integrate data stored in multiple Oracle and non-Oracle databases. Transparent remote data access with Distributed SQL allows grid users to run their applications against any other database without making any code change to the applications. While integrating data and managing transactions across multiple data stores, the Oracle database intelligently optimizes the execution plans to access data in the most efficient manner.

Ultra Large Database Support. For enabling Ultra Large Databases (ULDB), the Bigfile Tablespace feature allows Oracle Database to contain tablespaces made up of single large files rather than numerous smaller ones. This allows Oracle Database to utilize the ability of 64-bit systems to create and manage ultralarge files. The consequence of this is that Oracle Database can now scale up to 8 exabytes in size.

Managing the Grid

Because some of the key goals of a grid are to provide high availability, scalability, and service performance optimization with minimal costs and complexity, an integrated approach to grid management is necessary. These tools and features include:

Grid Management with Oracle Enterprise Manager. Enterprise Manager provides a simplified, centralized management framework for managing enterprise resources and analyzing a grid's performance. With Enterprise Manager, administrators can manage the grid environment through a Web browser throughout the system lifecycle, front to back, from any location on the network. With Oracle Database, this includes integrated management and monitoring of RAC databases as well as standby Data Guard systems, for failover or switchover scenarios.

Managing Security in the Grid

The dynamic environment in a grid makes security extremely important. Oracle makes managing security easy for you by centralizing security management for a distributed enterprise using the Lightweight Directory Access Protocol (LDAP)-compliant Oracle Internet Directory (OID). In this regard, Enterprise User Security provides the ability to create and manage privileges of a user globally—across all enterprise databases. The enterprise user privilege administration is done in the OID, thus avoiding the need to create the same user in multiple databases across a grid. Additionally, you now can store a Secure Sockets Layer (SSL) Certificate in a smart card, for roaming access to the grid.

Virtual Private Database. VPD provides server-enforced, fine-grained access control and a secure application context that can be used in a grid setting to enable multiple customers, partners, or departments utilizing the same database to have secure access to mission-critical data. VPD enables per-user and per-customer data access within a single database, with the assurance of physical data separation. VPD is enabled by associating one or more security policies with tables or views.

Overview of Application Architecture

There are two common ways to architect a database: client/server or multitier. As internet computing becomes more prevalent in computing environments, many database management systems are moving to a multitier environment.

Client/Server Architecture

Multiprocessing uses more than one processor for a set of related jobs. Distributed processing reduces the load on a single processor by allowing different processors to concentrate on a subset of related tasks, thus improving the performance and capabilities of the system as a whole.

An Oracle database system can easily take advantage of distributed processing by using its client/server architecture. In this architecture, the database system is divided into two parts: a front-end or a client, and a back-end or a server.

The Client

The client is a database application that initiates a request for an operation to be performed on the database server. It requests, processes, and presents data managed by the server. The client workstation can be optimized for its job. For example, it might not need large disk capacity, or it might benefit from graphic capabilities.

Often, the client runs on a different computer than the database server, generally on a PC. Many clients can simultaneously run against one server.

The Server

The server runs Oracle software and handles the functions required for concurrent, shared data access. The server receives and processes the SQL and PL/SQL statements that originate from client applications. The computer that manages the server can be optimized for its duties. For example, it can have large disk capacity and fast processors.

Multitier Architecture: Application Servers

A multitier architecture has the following components:

A client or initiator process that starts an operation

One or more application servers that perform parts of the operation. An application server provides access to the data for the client and performs some of the query processing, thus removing some of the load from the database server. It can serve as an interface between clients and multiple database servers, including providing an additional level of security.

An end or database server that stores most of the data used in the operation

This architecture enables use of an application server to do the following:

Validate the credentials of a client, such as a Web browser

Connect to an Oracle database server

Perform the requested operation on behalf of the client

If proxy authentication is being used, then the identity of the client is maintained throughout all tiers of the connection.

Overview of Physical Database Structures

The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files.

Datafiles

Every Oracle database has one or more physical datafiles. The datafiles contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database.

The characteristics of datafiles are:

A datafile can be associated with only one database.

Datafiles can have certain characteristics set to let them automatically extend when the database runs out of space.

One or more datafiles form a logical unit of database storage called a tablespace.

Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle. For example, assume that a user wants to access some data in a table of a database. If the requested information is not already in the memory cache for the database, then it is read from the appropriate datafiles and stored in memory.

Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once, as determined by the database writer process (DBWn) background process.

Control Files

Every Oracle database has a control file. A control file contains entries that specify the physical structure of the database. For example, it contains the following information:

Database name

Names and locations of datafiles and redo log files

Time stamp of database creation

Oracle can multiplex the control file, that is, simultaneously maintain a number of identical control file copies, to protect against a failure involving the control file.

Every time an instance of an Oracle database is started, its control file identifies the database and redo log files that must be opened for database operation to proceed. If the physical makeup of the database is altered (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle to reflect the change. A control file is also used in database recovery.

Redo Log Files

Every Oracle database has a set of two or more redo log files. The set of redo log files is collectively known as the redo log for the database. A redo log is made up of redo entries (also called redo records).

The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost.

To protect against a failure involving the redo log itself, Oracle allows a multiplexed redo log so that two or more copies of the redo log can be maintained on different disks.

The information in a redo log file is used only to recover the database from a system or media failure that prevents database data from being written to the datafiles. For example, if an unexpected power outage terminates database operation, then data in memory cannot be written to the datafiles, and the data is lost. However, lost data can be recovered when the database is opened, after power is restored. By applying the information in the most recent redo log files to the database datafiles, Oracle restores the database to the time at which the power failure occurred.

The process of applying the redo log during a recovery operation is called rolling forward.

Alert and Trace Log Files

Each server and background process can write to an associated trace file. When an internal error is detected by a process, it dumps information about the error to its trace file. Some of the information written to a trace file is intended for the database administrator, while other information is for Oracle Support Services. Trace file information is also used to tune applications and instances.

The alert file, or alert log, is a special trace file. The alert file of a database is a chronological log of messages and errors.

Overview of Logical Database Structures

The logical storage structures, including data blocks, extents, and segments, enable Oracle to have fine-grained control of disk space use.

Tablespaces

A database is divided into logical storage units called tablespaces, which group related logical structures together. For example, tablespaces commonly group together all application objects to simplify some administrative operations.

Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace. The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace.

Every Oracle database contains a SYSTEM tablespace and a SYSAUX tablespace. Oracle creates them automatically when the database is created. The system default is to create a smallfiletablespace, which is the traditional type of Oracle tablespace. The SYSTEM and SYSAUX tablespaces are created as smallfile tablespaces.

Oracle also lets you create bigfile tablespaces up to 8 exabytes (8 million terabytes) in size. With Oracle-managed files, bigfile tablespaces make datafiles completely transparent for users. In other words, you can perform operations on tablespaces, rather than the underlying datafiles.

Online and Offline Tablespaces

A tablespace can be online (accessible) or offline (not accessible). A tablespace is generally online, so that users can access the information in the tablespace. However, sometimes a tablespace is taken offline to make a portion of the database unavailable while allowing normal access to the remainder of the database. This makes many administrative tasks easier to perform.

Oracle Data Blocks

At the finest level of granularity, Oracle database data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk. The standard block size is specified by the DB_BLOCK_SIZE initialization parameter. In addition, you can specify up to five other block sizes. A database uses and allocates free database space in Oracle data blocks.

Extents

The next level of logical database space is an extent. An extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.

Segments

Above extents, the level of logical database storage is a segment. A segment is a set of extents allocated for a certain logical structure. The following table describes the different types of segments.

Segment

Description

Data segment

Each nonclustered table has a data segment. All table data is stored in the extents of the data segment.

For a partitioned table, each partition has a data segment.

Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.

Index segment

Each index has an index segment that stores all of its data.

For a partitioned index, each partition has an index segment.

Temporary segment

Temporary segments are created by Oracle when a SQL statement needs a temporary database area to complete execution. When the statement finishes execution, the extents in the temporary segment are returned to the system for future use.

Rollback segment

If you are operating in automatic undo management mode, then the database server manages undo space using tablespaces. Oracle recommends that you use automatic undo management.

Earlier releases of Oracle used rollback segments to store undo information. The information in a rollback segment was used during database recovery for generating read-consistent database information and for rolling back uncommitted transactions for users.

Space management for these rollback segments was complex, and Oracle has deprecated that method. This book discusses the undo tablespace method of managing undo; this eliminates the complexities of managing rollback segment space, and lets you exert control over how long undo is retained before being overwritten.

Oracle does use a SYSTEM rollback segment for performing system transactions. There is only one SYSTEM rollback segment and it is created automatically at CREATEDATABASE time and is always brought online at instance startup. You are not required to perform any operations to manage the SYSTEM rollback segment.

Oracle dynamically allocates space when the existing extents of a segment become full. In other words, when the extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.

Overview of Schemas and Common Schema Objects

A schema is a collection of database objects. A schema is owned by a database user and has the same name as that user. Schema objects are the logical structures that directly refer to the database's data. Schema objects include structures like tables, views, and indexes. (There is no relationship between a tablespace and a schema. Objects in the same schema can be in different tablespaces, and a tablespace can hold objects from different schemas.)

Some of the most common schema objects are defined in the following section.

Tables

Tables are the basic unit of data storage in an Oracle database. Database tables hold all user-accessible data. Each table has columns and rows. A table that has an employee database, for example, can have a column called employee number, and each row in that column is an employee's number.

Indexes

Indexes are optional structures associated with tables. Indexes can be created to increase the performance of data retrieval. Just as the index in this manual helps you quickly locate specific information, an Oracle index provides an access path to table data.

When processing a request, Oracle can use some or all of the available indexes to locate the requested rows efficiently. Indexes are useful when applications frequently query a table for a range of rows (for example, all employees with a salary greater than 1000 dollars) or a specific row.

Indexes are created on one or more columns of a table. After it is created, an index is automatically maintained and used by Oracle. Changes to table data (such as adding new rows, updating rows, or deleting rows) are automatically incorporated into all relevant indexes with complete transparency to the users.

Views

Views are customized presentations of data in one or more tables or other views. A view can also be considered a stored query. Views do not actually contain data. Rather, they derive their data from the tables on which they are based, referred to as the base tables of the views.

Like tables, views can be queried, updated, inserted into, and deleted from, with some restrictions. All operations performed on a view actually affect the base tables of the view.

Views provide an additional level of table security by restricting access to a predetermined set of rows and columns of a table. They also hide data complexity and store complex queries.

Clusters

Clusters are groups of one or more tables physically stored together because they share common columns and are often used together. Because related rows are physically stored together, disk access time improves.

Like indexes, clusters do not affect application design. Whether a table is part of a cluster is transparent to users and to applications. Data stored in a clustered table is accessed by SQL in the same way as data stored in a nonclustered table.

Synonyms

A synonym is an alias for any table, view, materialized view, sequence, procedure, function, package, type, Java class schema object, user-defined object type, or another synonym. Because a synonym is simply an alias, it requires no storage other than its definition in the data dictionary.

Overview of the Oracle Data Dictionary

Each Oracle database has a data dictionary. An Oracle data dictionary is a set of tables and views that are used as a read-only reference about the database. For example, a data dictionary stores information about both the logical and physical structure of the database. A data dictionary also stores the following information:

The valid users of an Oracle database

Information about integrity constraints defined for tables in the database

The amount of space allocated for a schema object and how much of it is in use

A data dictionary is created when a database is created. To accurately reflect the status of the database at all times, the data dictionary is automatically updated by Oracle in response to specific actions, such as when the structure of the database is altered. The database relies on the data dictionary to record, verify, and conduct ongoing work. For example, during database operation, Oracle reads the data dictionary to verify that schema objects exist and that users have proper access to them.

Overview of the Oracle Instance

An Oracle database server consists of an Oracle database and an Oracle instance. Every time a database is started, a system global area (SGA) is allocated and Oracle background processes are started. The combination of the background processes and memory buffers is called an Oracle instance.

Real Application Clusters: Multiple Instance Systems

Some hardware architectures (for example, shared disk systems) enable multiple computers to share access to data, software, or peripheral devices. Real Application Clusters (RAC) takes advantage of such architecture by running multiple instances that share a single physical database. In most applications, RAC enables access to a single database by users on multiple machines with increased performance.

An Oracle database server uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.

Instance Memory Structures

Oracle creates and uses memory structures to complete several jobs. For example, memory stores program code being run and data shared among users. Two basic memory structures are associated with Oracle: the system global area and the program global area. The following subsections explain each in detail.

System Global Area

The System Global Area (SGA) is a shared memory region that contains data and control information for one Oracle instance. Oracle allocates the SGA when an instance starts and deallocates it when the instance shuts down. Each instance has its own SGA.

Users currently connected to an Oracle database share the data in the SGA. For optimal performance, the entire SGA should be as large as possible (while still fitting in real memory) to store as much data in memory as possible and to minimize disk I/O.

Database Buffer Cache of the SGA

Database buffers store the most recently used blocks of data. The set of database buffers in an instance is the database buffer cache. The buffer cache contains modified as well as unmodified blocks. Because the most recently (and often, the most frequently) used data is kept in memory, less disk I/O is necessary, and performance is improved.

Redo Log Buffer of the SGA

The redo log buffer stores redo entries—a log of changes made to the database. The redo entries stored in the redo log buffers are written to an online redo log, which is used if database recovery is necessary. The size of the redo log is static.

Shared Pool of the SGA

The shared pool contains shared memory constructs, such as shared SQL areas. A shared SQL area is required to process every unique SQL statement submitted to a database. A shared SQL area contains information such as the parse tree and execution plan for the corresponding statement. A single shared SQL area is used by multiple applications that issue the same statement, leaving more shared memory for other uses.

Statement Handles or Cursors

A cursor is a handle or name for a private SQL area in which a parsed statement and other information for processing the statement are kept. (Oracle Call Interface, OCI, refers to these as statement handles.) Although most Oracle users rely on automatic cursor handling of Oracle utilities, the programmatic interfaces offer application designers more control over cursors.

For example, in precompiler application development, a cursor is a named resource available to a program and can be used specifically to parse SQL statements embedded within the application. Application developers can code an application so it controls the phases of SQL statement execution and thus improves application performance.

Program Global Area

The Program Global Area (PGA) is a memory buffer that contains data and control information for a server process. A PGA is created by Oracle when a server process is started. The information in a PGA depends on the Oracle configuration.

Oracle Background Processes

An Oracle database uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.

The architectural features discussed in this section enable the Oracle database to support:

Many users concurrently accessing a single database

The high performance required by concurrent multiuser, multiapplication database systems

Oracle creates a set of background processes for each instance. The background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. They asynchronously perform I/O and monitor other Oracle process to provide increased parallelism for better performance and reliability.

There are numerous background processes, and each Oracle instance can use several background processes.

Process Architecture

A process is a "thread of control" or a mechanism in an operating system that can run a series of steps. Some operating systems use the terms job or task. A process generally has its own private memory area in which it runs.

An Oracle database server has two general types of processes: user processes and Oracle processes.

User (Client) Processes

User processes are created and maintained to run the software code of an application program (such as an OCI or OCCI program) or an Oracle tool (such as Enterprise Manager). User processes also manage communication with the server process through the program interface, which is described in a later section.

Oracle Processes

Oracle processes are invoked by other processes to perform functions on behalf of the invoking process.

Oracle creates server processes to handle requests from connected user processes. A server process communicates with the user process and interacts with Oracle to carry out requests from the associated user process. For example, if a user queries some data not already in the database buffers of the SGA, then the associated server process reads the proper data blocks from the datafiles into the SGA.

Oracle can be configured to vary the number of user processes for each server process. In a dedicated server configuration, a server process handles requests for a single user process. A shared server configuration lets many user processes share a small number of server processes, minimizing the number of server processes and maximizing the use of available system resources.

On some systems, the user and server processes are separate, while on others they are combined into a single process. If a system uses the shared server or if the user and server processes run on different machines, then the user and server processes must be separate. Client/server systems separate the user and server processes and run them on different machines.

Overview of Accessing the Database

This section describes Oracle Net Services, as well as how to start up the database.

Network Connections

Oracle Net Services is Oracle's mechanism for interfacing with the communication protocols used by the networks that facilitate distributed processing and distributed databases.

Communication protocols define the way that data is transmitted and received on a network. Oracle Net Services supports communications on all major network protocols, including TCP/IP, HTTP, FTP, and WebDAV.

Using Oracle Net Services, application developers do not need to be concerned with supporting network communications in a database application. If a new protocol is used, then the database administrator makes some minor changes, while the application requires no modifications and continues to function.

Oracle Net, a component of Oracle Net Services, enables a network session from a client application to an Oracle database server. Once a network session is established, Oracle Net acts as the data courier for both the client application and the database server. It establishes and maintains the connection between the client application and database server, as well as exchanges messages between them. Oracle Net can perform these jobs because it is located on each computer in the network.

Starting Up the Database

The three steps to starting an Oracle database and making it available for systemwide use are:

Start an instance.

Mount the database.

Open the database.

A database administrator can perform these steps using the SQL*Plus STARTUP statement or Enterprise Manager. When Oracle starts an instance, it reads the server parameter file (SPFILE) or initialization parameter file to determine the values of initialization parameters. Then, it allocates an SGA, and creates background processes.

How Oracle Works

The following example describes the most basic level of operations that Oracle performs. This illustrates an Oracle configuration where the user and associated server process are on separate machines (connected through a network).

An instance has started on the computer running Oracle (often called the host or database server).

A computer running an application (a local machine or client workstation) runs the application in a user process. The client application attempts to establish a connection to the server using the proper Oracle Net Services driver.

The server is running the proper Oracle Net Services driver. The server detects the connection request from the application and creates a dedicated server process on behalf of the user process.

The user runs a SQL statement and commits the transaction. For example, the user changes a name in a row of a table.

The server process receives the statement and checks the shared pool for any shared SQL area that contains a similar SQL statement. If a shared SQL area is found, then the server process checks the user's access privileges to the requested data, and the previously existing shared SQL area is used to process the statement. If not, then a new shared SQL area is allocated for the statement, so it can be parsed and processed.

The server process retrieves any necessary data values from the actual datafile (table) or those stored in the SGA.

The server process modifies data in the system global area. The DBWn process writes modified blocks permanently to disk when doing so is efficient. Because the transaction is committed, the LGWR process immediately records the transaction in the redo log file.

If the transaction is successful, then the server process sends a message across the network to the application. If it is not successful, then an error message is transmitted.

Throughout this entire procedure, the other background processes run, watching for conditions that require intervention. In addition, the database server manages other users' transactions and prevents contention between transactions that request the same data.

Concurrency

A primary concern of a multiuser database management system is how to control concurrency, which is the simultaneous access of the same data by many users. Without adequate concurrency controls, data could be updated or changed improperly, compromising data integrity.

One way to manage data concurrency is to make each user wait for a turn. The goal of a database management system is to reduce that wait so it is either nonexistent or negligible to each user. All data manipulation language statements should proceed with as little interference as possible, and destructive interactions between concurrent transactions must be prevented. Destructive interaction is any interaction that incorrectly updates data or incorrectly alters underlying data structures. Neither performance nor data integrity can be sacrificed.

Oracle resolves such issues by using various types of locks and a multiversion consistency model. These features are based on the concept of a transaction. It is the application designer's responsibility to ensure that transactions fully exploit these concurrency and consistency features.

Read Consistency

Read consistency, as supported by Oracle, does the following:

Guarantees that the set of data seen by a statement is consistent with respect to a single point in time and does not change during statement execution (statement-level read consistency)

Ensures that readers of database data do not wait for writers or other readers of the same data

Ensures that writers of database data do not wait for readers of the same data

Ensures that writers only wait for other writers if they attempt to update identical rows in concurrent transactions

The simplest way to think of Oracle's implementation of read consistency is to imagine each user operating a private copy of the database, hence the multiversion consistency model.

Read Consistency, Undo Records, and Transactions

To manage the multiversion consistency model, Oracle must create a read-consistent set of data when a table is queried (read) and simultaneously updated (written). When an update occurs, the original data values changed by the update are recorded in the database undo records. As long as this update remains part of an uncommitted transaction, any user that later queries the modified data views the original data values. Oracle uses current information in the system global area and information in the undo records to construct a read-consistent view of a table's data for a query.

Only when a transaction is committed are the changes of the transaction made permanent. Statements that start after the user's transaction is committed only see the changes made by the committed transaction.

The transaction is key to Oracle's strategy for providing read consistency. This unit of committed (or uncommitted) SQL statements:

Dictates the start point for read-consistent views generated on behalf of readers

Controls when modified data can be seen by other transactions of the database for reading or updating

Read-Only Transactions

By default, Oracle guarantees statement-level read consistency. The set of data returned by a single query is consistent with respect to a single point in time. However, in some situations, you might also require transaction-level read consistency. This is the ability to run multiple queries within a single transaction, all of which are read-consistent with respect to the same point in time, so that queries in this transaction do not see the effects of intervening committed transactions. If you want to run a number of queries against multiple tables and if you are not doing any updating, you prefer a read-only transaction.

Locking Mechanisms

Oracle also uses locks to control concurrent access to data. When updating information, the data server holds that information with a lock until the update is submitted or committed. Until that happens, no one else can make changes to the locked information. This ensures the data integrity of the system.

Oracle provides unique non-escalating row-level locking. Unlike other data servers that "escalate" locks to cover entire groups of rows or even the entire table, Oracle always locks only the row of information being updated. Because Oracle includes the locking information with the actual rows themselves, Oracle can lock an unlimited number of rows so users can work concurrently without unnecessary delays.

Automatic Locking

Oracle locking is performed automatically and requires no user action. Implicit locking occurs for SQL statements as necessary, depending on the action requested. Oracle's lock manager automatically locks table data at the row level. By locking table data at the row level, contention for the same data is minimized.

Oracle's lock manager maintains several different types of row locks, depending on what type of operation established the lock. The two general types of locks are exclusive locks and share locks. Only one exclusive lock can be placed on a resource (such as a row or a table); however, many share locks can be placed on a single resource. Both exclusive and share locks always allow queries on the locked resource but prohibit other activity on the resource (such as updates and deletes).

Manual Locking

Under some circumstances, a user might want to override default locking. Oracle allows manual override of automatic locking features at both the row level (by first querying for the rows that will be updated in a subsequent statement) and the table level.

Quiesce Database

Database administrators occasionally need isolation from concurrent non-database administrator actions, that is, isolation from concurrent non-database administrator transactions, queries, or PL/SQL statements. One way to provide such isolation is to shut down the database and reopen it in restricted mode. You could also put the system into quiesced state without disrupting users. In quiesced state, the database administrator can safely perform certain actions whose executions require isolation from concurrent non-DBA users.

Real Application Clusters

Real Application Clusters (RAC) comprises several Oracle instances running on multiple clustered machines, which communicate with each other by means of a so-called interconnect. RAC uses cluster software to access a shared database that resides on shared disk. RAC combines the processing power of these multiple interconnected computers to provide system redundancy, near linear scalability, and high availability. RAC also offers significant advantages for both OLTP and data warehouse systems and all systems and applications can efficiently exploit clustered environments.

You can scale applications in RAC environments to meet increasing data processing demands without changing the application code. As you add resources such as nodes or storage, RAC extends the processing powers of these resources beyond the limits of the individual components.

Portability

Oracle provides unique portability across all major platforms and ensures that your applications run without modification after changing platforms. This is because the Oracle code base is identical across platforms, so you have identical feature functionality across all platforms, for complete application transparency. Because of this portability, you can easily upgrade to a more powerful server as your requirements change.

Overview of Manageability Features

People who administer the operation of an Oracle database system, known as database administrators (DBAs), are responsible for creating Oracle databases, ensuring their smooth operation, and monitoring their use. In addition to the many alerts and advisors Oracle provides, Oracle also offers the following features:

Automatic Storage Management

Automatic Storage Management automates and simplifies the layout of datafiles, control files, and log files. Database files are automatically distributed across all available disks, and database storage is rebalanced whenever the storage configuration changes. It provides redundancy through the mirroring of database files, and it improves performance by automatically distributing database files across all available disks. Rebalancing of the database's storage automatically occurs whenever the storage configuration changes.

The Scheduler

To help simplify management tasks, as well as providing a rich set of functionality for complex scheduling needs, Oracle provides a collection of functions and procedures in the DBMS_SCHEDULER package. Collectively, these functions are called the Scheduler, and they are callable from any PL/SQL program.

The Scheduler lets database administrators and application developers control when and where various tasks take place in the database environment. For example, database administrators can schedule and monitor database maintenance jobs such as backups or nightly data warehousing loads and extracts.

Database Resource Manager

Traditionally, the operating systems regulated resource management among the various applications running on a system, including Oracle databases. The Database Resource Manager controls the distribution of resources among various sessions by controlling the execution schedule inside the database. By controlling which sessions run and for how long, the Database Resource Manager can ensure that resource distribution matches the plan directive and hence, the business objectives.

Overview of Database Backup and Recovery Features

In every database system, the possibility of a system or hardware failure always exists. If a failure occurs and affects the database, then the database must be recovered. The goals after a failure are to ensure that the effects of all committed transactions are reflected in the recovered database and to return to normal operation as quickly as possible while insulating users from problems caused by the failure.

Oracle provides various mechanisms for the following:

Database recovery required by different types of failures

Flexible recovery operations to suit any situation

Availability of data during backup and recovery operations so users of the system can continue to work

Types of Failures

Several circumstances can halt the operation of an Oracle database. The most common types of failure are described in the following table.

Failure

Description

User error

Requires a database to be recovered to a point in time before the error occurred. For example, a user could accidentally drop a table. To enable recovery from user errors and accommodate other unique recovery requirements, Oracle provides exact point-in-time recovery. For example, if a user accidentally drops a table, the database can be recovered to the instant in time before the table was dropped.

Statement failure

Occurs when there is a logical failure in the handling of a statement in an Oracle program. When statement failure occurs, any effects of the statement are automatically undone by Oracle and control is returned to the user.

Process failure

Results from a failure in a user process accessing Oracle, such as an abnormal disconnection or process termination. The background process PMON automatically detects the failed user process, rolls back the uncommitted transaction of the user process, and releases any resources that the process was using.

Instance failure

Occurs when a problem arises that prevents an instance from continuing work. Instance failure can result from a hardware problem such as a power outage, or a software problem such as an operating system failure. When an instance failure occurs, the data in the buffers of the system global area is not written to the datafiles.

After an instance failure, Oracle automatically performs instance recovery. If one instance in a Real Application Clusters (RAC) environment, another instance recovers the redo for the failed instance. In a single-instance database, or in a RAC database in which all instances fail, Oracle automatically applies all redo when you restart the database.

Media (disk) failure

An error can occur when trying to write or read a file on disk that is required to operate the database. A common example is a disk head failure, which causes the loss of all files on a disk drive.

Different files can be affected by this type of disk failure, including the datafiles, the redo log files, and the control files. Also, because the database instance cannot continue to function properly, the data in the database buffers of the system global area cannot be permanently written to the datafiles.

A disk failure requires you to restore lost files and then perform media recovery. Unlike instance recovery, media recovery must be initiated by the user. Media recovery updates restored datafiles so the information in them corresponds to the most recent time point before the disk failure, including the committed data in memory that was lost because of the failure.

Oracle provides for complete media recovery from all possible types of hardware failures, including disk failures. Options are provided so that a database can be completely recovered or partially recovered to a specific point in time.

If some datafiles are damaged in a disk failure but most of the database is intact and operational, the database can remain open while the required tablespaces are individually recovered. Therefore, undamaged portions of a database are available for normal use while damaged portions are being recovered.

Structures Used for Recovery

Oracle uses several structures to provide complete recovery from an instance or disk failure: the redo log, undo records, a control file, and database backups.

The Redo Log

The redo log is a set of files that protect altered database data in memory that has not been written to the datafiles. The redo log can consist of the online redo log and the archived redo log.

Note:

Because the online redo log is always online, as opposed to an archived copy of a redo log, thus it is usually referred to as simply "the redo log".

The online redo log is a set of two or more online redo log files that record all changes made to the database, including uncommitted and committed changes. Redo entries are temporarily stored in redo log buffers of the system global area, and the background process LGWR writes the redo entries sequentially to an online redo log file. LGWR writes redo entries continually, and it also writes a commit record every time a user process commits a transaction.

Optionally, filled online redo files can be manually or automatically archived before being reused, creating archived redo logs. To enable or disable archiving, set the database in one of the following modes:

ARCHIVELOG: The filled online redo log files are archived before they are reused in the cycle.

NOARCHIVELOG: The filled online redo log files are not archived.

In ARCHIVELOG mode, the database can be completely recovered from both instance and disk failure. The database can also be backed up while it is open and available for use. However, additional administrative operations are required to maintain the archived redo log.

If the database redo log operates in NOARCHIVELOG mode, then the database can be completely recovered from instance failure, but not from disk failure. Also, the database can be backed up only while it is completely closed. Because no archived redo log is created, no extra work is required by the database administrator.

Undo Records

Undo records are stored in undo tablespaces. Oracle uses the undo data for a variety of purposes, including accessing before-images of blocks changed in uncommitted transactions. During database recovery, Oracle applies all changes recorded in the redo log and then uses undo information to roll back any uncommitted transactions.

Control Files

The control files include information about the file structure of the database and the current log sequence number being written by LGWR. During normal recovery procedures, the information in a control file guides the automatic progression of the recovery operation.

Database Backups

Because one or more files can be physically damaged as the result of a disk failure, media recovery requires the restoration of the damaged files from the most recent operating system backup of a database. You can either back up the database files with Recovery Manager (RMAN), or use operating system utilities. RMAN is an Oracle utility that manages backup and recovery operations, creates backups of database files (datafiles, control files, and archived redo log files), and restores or recovers a database from backups.

Overview of High Availability Features

Computing environments configured to provide nearly full-time availability are known as high availability systems. Such systems typically have redundant hardware and software that makes the system available despite failures. Well-designed high availability systems avoid having single points-of-failure.

When failures occur, the fail over process moves processing performed by the failed component to the backup component. This process remasters systemwide resources, recovers partial or failed transactions, and restores the system to normal, preferably within a matter of microseconds. The more transparent that fail over is to users, the higher the availability of the system.

Oracle has a number of products and features that provide high availability in cases of unplanned downtime or planned downtime. These include Fast-Start Fault Recovery, Real Application Clusters, Recovery Manager (RMAN), backup and recovery solutions, Oracle Flashback, partitioning, Oracle Data Guard, LogMiner, multiplexed redo log files, online reorganization. These can be used in various combinations to meet specific high availability needs.

Overview of Business Intelligence Features

This section describes several business intelligence features.

Data Warehousing

A data warehouse is a relational database designed for query and analysis rather than for transaction processing. It usually contains historical data derived from transaction data, but it can include data from other sources. It separates analysis workload from transaction workload and enables an organization to consolidate data from several sources.

In addition to a relational database, a data warehouse environment includes an extraction, transportation, transformation, and loading (ETL) solution, an online analytical processing (OLAP) engine, client analysis tools, and other applications that manage the process of gathering data and delivering it to business users.

Extraction, Transformation, and Loading (ETL)

You must load your data warehouse regularly so that it can serve its purpose of facilitating business analysis. To do this, data from one or more operational systems must be extracted and copied into the warehouse. The process of extracting data from source systems and bringing it into the data warehouse is commonly called ETL, which stands for extraction, transformation, and loading.

Materialized Views

A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, a materialized view contains the rows resulting from a query against one or more base tables or views. A materialized view can be stored in the same database as its base tables or in a different database.

Materialized views stored in the same database as their base tables can improve query performance through query rewrites. Query rewrites are particularly useful in a data warehouse environment.

Bitmap Indexes in Data Warehousing

Data warehousing environments typically have large amounts of data and ad hoc queries, but a low level of concurrent database manipulation language (DML) transactions. For such applications, bitmap indexing provides:

Reduced response time for large classes of ad hoc queries

Reduced storage requirements compared to other indexing techniques

Dramatic performance gains even on hardware with a relatively small number of CPUs or a small amount of memory

Efficient maintenance during parallel DML and loads

Fully indexing a large table with a traditional B-tree index can be prohibitively expensive in terms of space because the indexes can be several times larger than the data in the table. Bitmap indexes are typically only a fraction of the size of the indexed data in the table.

Table Compression

To reduce disk use and memory use (specifically, the buffer cache), you can store tables and partitioned tables in a compressed format inside the database. This often leads to a better scaleup for read-only operations. Table compression can also speed up query execution. There is, however, a slight cost in CPU overhead.

Parallel Execution

When Oracle runs SQL statements in parallel, multiple processes work together simultaneously to run a single SQL statement. By dividing the work necessary to run a statement among multiple processes, Oracle can run the statement more quickly than if only a single process ran it. This is called parallelexecution or parallelprocessing.

Parallel execution dramatically reduces response time for data-intensive operations on large databases, because statement processing can be split up among many CPUs on a single Oracle system.

Analytic SQL

Oracle has many SQL operations for performing analytic operations in the database. These include ranking, moving averages, cumulative sums, ratio-to-reports, and period-over-period comparisons.

Oracle OLAP provides the query performance and calculation capability previously found only in multidimensional databases to Oracle's relational platform. In addition, it provides a Java OLAP API that is appropriate for the development of internet-ready analytical applications. Unlike other combinations of OLAP and RDBMS technology, Oracle OLAP is not a multidimensional database using bridges to move data from the relational data store to a multidimensional data store. Instead, it is truly an OLAP-enabled relational database. As a result, Oracle provides the benefits of a multidimensional database along with the scalability, accessibility, security, manageability, and high availability of the Oracle database. The Java OLAP API, which is specifically designed for internet-based analytical applications, offers productive data access.

Data Mining

With Oracle Data Mining, data never leaves the database — the data, data preparation, model building, and model scoring results all remain in the database. This enables Oracle to provide an infrastructure for application developers to integrate data mining seamlessly with database applications. Some typical examples of the applications that data mining are used in are call centers, ATMs, ERM, and business planning applications. Data mining functions such as model building, testing, and scoring are provided through a Java API.

Partitioning

Partitioning addresses key issues in supporting very large tables and indexes by letting you decompose them into smaller and more manageable pieces called partitions. SQL queries and DML statements do not need to be modified in order to access partitioned tables. However, after partitions are defined, DDL statements can access and manipulate individuals partitions rather than entire tables or indexes. This is how partitioning can simplify the manageability of large database objects. Also, partitioning is entirely transparent to applications.

Partitioning is useful for many different types of applications, particularly applications that manage large volumes of data. OLTP systems often benefit from improvements in manageability and availability, while data warehousing systems benefit from performance and manageability.

Overview of Content Management Features

Oracle includes datatypes to handle all the types of rich Internet content such as relational data, object-relational data, XML, text, audio, video, image, and spatial. These datatypes appear as native types in the database. They can all be queried using SQL. A single SQL statement can include data belonging to any or all of these datatypes.

XML in Oracle

XML, eXtensible Markup Language, is the standard way to identify and describe data on the Web. Oracle XML DB treats XML as a native datatype in the database. Oracle XML DB offers a number of easy ways to create XML documents from relational tables. The result of any SQL query can be automatically converted into an XML document. Oracle also includes a set of utilities, available in Java and C++, to simplify the task of creating XML documents.

Oracle includes five XML developer's kits, or XDKs. Each consists of a standards-based set of components, tools, and utilities. The XDKs are available for Java, C, C++, PL/SQL, and Java Beans.

LOBs

The LOB datatypes BLOB, CLOB, NCLOB, and BFILE enable you to store and manipulate large blocks of unstructured data (such as text, graphic images, video clips, and sound waveforms) in binary or character format. They provide efficient, random, piece-wise access to the data.

Oracle Text

Oracle Text indexes any document or textual content to add fast, accurate retrieval of information. Oracle Text allows text searches to be combined with regular database searches in a single SQL statement. The ability to find documents based on their textual content, metadata, or attributes, makes the Oracle Database the single point of integration for all data management.

The Oracle Text SQL API makes it simple and intuitive for application developers and DBAs to create and maintain Text indexes and run Text searches.

Oracle interMedia

Oracle interMedia provides an array of services to develop and deploy traditional, Web, and wireless applications that include image, audio, and video in an integrated fashion. Multimedia content can be stored and managed directly in Oracle, or Oracle can store and index metadata together with external references that enable efficient access to media content stored outside the database.

Oracle Spatial

Oracle includes built-in spatial features that let you store, index, and manage location content (assets, buildings, roads, land parcels, sales regions, and so on.) and query location relationships using the power of the database. The Oracle Spatial Option adds advanced spatial features such as linear reference support and coordinate systems.

Overview of Security Features

Oracle includes security features that control how a database is accessed and used. For example, security mechanisms:

Prevent unauthorized database access

Prevent unauthorized access to schema objects

Audit user actions

Associated with each database user is a schema by the same name. By default, each database user creates and has access to all objects in the corresponding schema.

Database security can be classified into two categories: system security and data security.

System security includes the mechanisms that control the access and use of the database at the system level. For example, system security includes:

Valid user name/password combinations

The amount of disk space available to a user's schema objects

The resource limits for a user

System security mechanisms check whether a user is authorized to connect to the database, whether database auditing is active, and which system operations a user can perform.

Data security includes the mechanisms that control the access and use of the database at the schema object level. For example, data security includes:

Which users have access to a specific schema object and the specific types of actions allowed for each user on the schema object (for example, user SCOTT can issue SELECT and INSERT statements but not DELETE statements using the employees table)

Security Mechanisms

The Oracle database provides discretionary access control, which is a means of restricting access to information based on privileges. The appropriate privilege must be assigned to a user in order for that user to access a schema object. Appropriately privileged users can grant other users privileges at their discretion.

Oracle manages database security using several different facilities:

Authentication to validate the identity of the entities using your networks, databases, and applications

Authorization processes to limit access and actions, limits that are linked to user's identities and roles.

Overview of Data Integrity and Triggers

Data must adhere to certain business rules, as determined by the database administrator or application developer. For example, assume that a business rule says that no row in the inventory table can contain a numeric value greater than nine in the sale_discount column. If an INSERT or UPDATE statement attempts to violate this integrity rule, then Oracle must undo the invalid statement and return an error to the application. Oracle provides integrity constraints and database triggers to manage data integrity rules.

Note:

Database triggers let you define and enforce integrity rules, but a database trigger is not the same as an integrity constraint. Among other things, a database trigger does not check data already loaded into a table. Therefore, it is strongly recommended that you use database triggers only when the integrity rule cannot be enforced by integrity constraints.

Integrity Constraints

An integrity constraint is a declarative way to define a business rule for a column of a table. An integrity constraint is a statement about table data that is always true and that follows these rules:

If an integrity constraint is created for a table and some existing table data does not satisfy the constraint, then the constraint cannot be enforced.

After a constraint is defined, if any of the results of a DML statement violate the integrity constraint, then the statement is rolled back, and an error is returned.

Integrity constraints are defined with a table and are stored as part of the table's definition in the data dictionary, so that all database applications adhere to the same set of rules. When a rule changes, it only needs be changed once at the database level and not many times for each application.

The following integrity constraints are supported by Oracle:

NOTNULL: Disallows nulls (empty entries) in a table's column.

UNIQUEKEY: Disallows duplicate values in a column or set of columns.

PRIMARYKEY: Disallows duplicate values and nulls in a column or set of columns.

FOREIGNKEY: Requires each value in a column or set of columns to match a value in a related table's UNIQUE or PRIMARYKEY. FOREIGNKEY integrity constraints also define referential integrity actions that dictate what Oracle should do with dependent data if the data it references is altered.

CHECK: Disallows values that do not satisfy the logical expression of the constraint.

Keys

Key is used in the definitions of several types of integrity constraints. A key is the column or set of columns included in the definition of certain types of integrity constraints. Keys describe the relationships between the different tables and columns of a relational database. Individual values in a key are called key values.

The different types of keys include:

Primary key: The column or set of columns included in the definition of a table's PRIMARYKEY constraint. A primary key's values uniquely identify the rows in a table. Only one primary key can be defined for each table.

Unique key: The column or set of columns included in the definition of a UNIQUE constraint.

Foreign key: The column or set of columns included in the definition of a referential integrity constraint.

Referenced key: The unique key or primary key of the same or a different table referenced by a foreign key.

Triggers

Triggers are procedures written in PL/SQL, Java, or C that run (fire) implicitly whenever a table or view is modified or when some user actions or database system actions occur.

Triggers supplement the standard capabilities of Oracle to provide a highly customized database management system. For example, a trigger can restrict DML operations against a table to those issued during regular business hours.

Overview of Information Integration Features

A distributed environment is a network of disparate systems that seamlessly communicate with each other. Each system in the distributed environment is called a node. The system to which a user is directly connected is called the local system. Any additional systems accessed by this user are called remote systems. A distributed environment allows applications to access and exchange data from the local and remote systems. All the data can be simultaneously accessed and modified.

Distributed SQL

A homogeneous distributed database system is a network of two or more Oracle databases that reside on one or more machines. Distributed SQL enables applications and users to simultaneously access or modify the data in several databases as easily as they access or modify a single database.

An Oracle distributed database system can be transparent to users, making it appear as though it is a single Oracle database. Companies can use this distributed SQL feature to make all its Oracle databases look like one and thus reduce some of the complexity of the distributed system.

Oracle uses database links to enable users on one database to access objects in a remote database. A local user can access a link to a remote database without having to be a user on the remote database.

Location Transparency

Location transparency occurs when the physical location of data is transparent to the applications and users. For example, a view that joins table data from several databases provides location transparency because the user of the view does not need to know from where the data originates.

SQL and Transaction Transparency

Oracle's provides query, update, and transaction transparency. For example, standard SQL statements like SELECT, INSERT, UPDATE, and DELETE work just as they do in a non-distributed database environment. Additionally, applications control transactions using the standard SQL statements COMMIT, SAVEPOINT, and ROLLBACK. Oracle ensures the integrity of data in a distributed transaction using the two-phase commit mechanism.

Distributed Query Optimization

Distributed query optimization reduces the amount of data transfer required between sites when a transaction retrieves data from remote tables referenced in a distributed SQL statement.

Oracle Streams

Oracle Streams enables the propagation and management of data, transactions, and events in a data stream either within a database, or from one database to another. The stream routes published information to subscribed destinations. As users' needs change, they can simply implement a new capability of Oracle Streams, without sacrificing existing capabilities.

Oracle Streams provides a set of elements that lets users control what information is put into a stream, how the stream flows or is routed from node to node, what happens to events in the stream as they flow into each node, and how the stream terminates. By specifying the configuration of the elements acting on the stream, a user can address specific requirements, such as message queuing or data replication.

Capture

Oracle Streams implicitly and explicitly captures events and places them in the staging area. Database events, such as DML and DDL, are implicitly captured by mining the redo log files. Sophisticated subscription rules can determine what events should be captured.

Staging

The staging area is a queue that provides a service to store and manage captured events. Changes to database tables are formatted as logical change records (LCR), and stored in a staging area until subscribers consume them. LCR staging provides a holding area with security, as well as auditing and tracking of LCR data.

Consumption

Messages in a staging area are consumed by the apply engine, where changes are applied to a database or consumed by an application. A flexible apply engine allows use of a standard or custom apply function. Support for explicit dequeue lets application developers use Oracle Streams to reliably exchange messages. They can also notify applications of changes to data, by still leveraging the change capture and propagation features of Oracle Streams.

Message Queuing

Oracle Streams Advanced Queuing is built on top of the flexible Oracle Streams infrastructure. It provides a unified framework for processing events. Events generated in applications, in workflow, or implicitly captured from redo logs or database triggers can be captured in a queue. These events can be consumed in a variety of ways. They can be automatically applied with a user-defined function or database table operation, can be explicitly dequeued, or a notification can be sent to the consuming application. These events can be transformed at any stage. If the consuming application is on a different database, then the events are automatically propagated to the appropriate database. Operations on these events can be automatically audited, and the history can be retained for the user-specified duration.

Data Replication

Replication is the maintenance of database objects in two or more databases. Oracle Streams provides powerful replication features that can be used to keep multiple copies of distributed objects synchronized.

Oracle Streams automatically determines what information is relevant and shares that information with those who need it. This active sharing of information includes capturing and managing events in the database including data changes with DML and propagating those events to other databases and applications. Data changes can be applied directly to the replica database, or can call a user-defined procedure to perform alternative work at the destination database, for example, populate a staging table used to load a data warehouse.

Oracle Streams is an open information sharing solution, supporting heterogeneous replication between Oracle and non-Oracle systems. Using a transparent gateway, DML changes initiated at Oracle databases can be applied to non-Oracle platforms.

Oracle Streams is fully inter-operational with materialized views, or snapshots, which can maintain updatable or read-only, point-in-time copies of data. They can contain a full copy of a table or a defined subset of the rows in the master table that satisfy a value-based selection criterion. There can be multitier materialized views as well, where one materialized view is a subset of another materialized view. Materialized views are periodically updated, or refreshed, from their associated master tables through transactionally consistent batch updates.

They translate third party SQL dialects, data dictionaries, and datatypes into Oracle formats, thus making the non-Oracle data store appear as a remote Oracle database. These technologies enable companies to seamlessly integrate the different systems and provide a consolidated view of the company as a whole.

Oracle Transparent Gateways and Generic Connectivity can be used for synchronous access, using distributed SQL, and for asynchronous access, using Oracle Streams. Introducing a Transparent Gateway into an Oracle Streams environment enables replication of data from an Oracle database to a non-Oracle database.

Generic Connectivity is a generic solution, while Oracle Transparent Gateways are tailored solutions, specifically coded for the non-Oracle system.

Oracle Database Application Development

SQL and PL/SQL form the core of Oracle's application development stack. Not only do most enterprise back-ends run SQL, but Web applications accessing databases do so using SQL (wrappered by Java classes as JDBC), Enterprise Application Integration applications generate XML from SQL queries, and content-repositories are built on top of SQL tables. It is a simple, widely understood, unified data model. It is used standalone in many applications, but it is also invoked directly from Java (JDBC), Oracle Call Interface (OCI), Oracle C++ Call Interface (OCCI), or XSU (XML SQL Utility). Stored packages, procedures, and triggers can all be written in PL/SQL or in Java.

Overview of Oracle SQL

SQL (pronounced SEQUEL) is the programming language that defines and manipulates the database. SQL databases are relational databases, which means that data is stored in a set of simple relations.

SQL Statements

All operations on the information in an Oracle database are performed using SQL statements. A SQL statement is a string of SQL text. A statement must be the equivalent of a complete SQL sentence, as in:

SELECT last_name, department_id FROM employees;

Only a complete SQL statement can run successfully. A sentence fragment, such as the following, generates an error indicating that more text is required:

SELECT last_name

A SQL statement can be thought of as a very simple, but powerful, computer program or instruction. SQL statements are divided into the following categories:

Data Definition Language (DDL) Statements

These statements create, alter, maintain, and drop schema objects. DDL statements also include statements that permit a user to grant other users the privileges to access the database and specific objects within the database.

Data Manipulation Language (DML) Statements

These statements manipulate data. For example, querying, inserting, updating, and deleting rows of a table are all DML operations. The most common SQL statement is the SELECT statement, which retrieves data from the database. Locking a table or view and examining the execution plan of a SQL statement are also DML operations.

Transaction Control Statements

These statements manage the changes made by DML statements. They enable a user to group changes into logical transactions. Examples include COMMIT, ROLLBACK, and SAVEPOINT.

Session Control Statements

These statements let a user control the properties of the current session, including enabling and disabling roles and changing language settings. The two session control statements are ALTERSESSION and SETROLE.

System Control Statements

These statements change the properties of the Oracle database instance. The only system control statement is ALTERSYSTEM. It lets users change settings, such as the minimum number of shared servers, kill a session, and perform other tasks.

Embedded SQL Statements

These statements incorporate DDL, DML, and transaction control statements in a procedural language program, such as those used with the Oracle precompilers. Examples include OPEN, CLOSE, FETCH, and EXECUTE.

Overview of PL/SQL

PL/SQL is Oracle's procedural language extension to SQL. PL/SQL combines the ease and flexibility of SQL with the procedural functionality of a structured programming language, such as IF ... THEN, WHILE, and LOOP.

When designing a database application, consider the following advantages of using stored PL/SQL:

PL/SQL code can be stored centrally in a database. Network traffic between applications and the database is reduced, so application and system performance increases. Even when PL/SQL is not stored in the database, applications can send blocks of PL/SQL to the database rather than individual SQL statements, thereby reducing network traffic.

Data access can be controlled by stored PL/SQL code. In this case, PL/SQL users can access data only as intended by application developers, unless another access route is granted.

PL/SQL blocks can be sent by an application to a database, running complex operations without excessive network traffic.

PL/SQL Program Units

Procedures and functions are sets of SQL and PL/SQL statements grouped together as a unit to solve a specific problem or to perform a set of related tasks. They are created and stored in compiled form in the database and can be run by a user or a database application.

Procedures and functions are identical, except that functions always return a single value to the user. Procedures do not return values.

Packages encapsulate and store related procedures, functions, variables, and other constructs together as a unit in the database. They offer increased functionality (for example, global package variables can be declared and used by any procedure in the package). They also improve performance (for example, all objects of the package are parsed, compiled, and loaded into memory once).

Overview of Application Programming Languages (APIs)

Oracle Database developers have a choice of languages for developing applications—C, C++, Java, COBOL, PL/SQL, and Visual Basic. The entire functionality of the database is available in all the languages. All language-specific standards are supported. Developers can choose the languages in which they are most proficient or one that is most suitable for a specific task. For example an application might use Java on the server side to create dynamic Web pages, PL/SQL to implement stored procedures in the database, and C++ to implement computationally intensive logic in the middle tier.

The Oracle Call Interface (OCI) is a C data access API for Oracle Database. It supports the entire Oracle Database feature set. Many data access APIs, such as OCCI, ODBC, Oracle JDBC Type2 drivers, and so on, are built on top of OCI. OCI provides powerful functionality to build high performance, secure, scalable, and fault-tolerant applications. OCI is also used within the server for the data access needs of database kernel components, along with distributed database access. OCI lets an application developer use C function calls to access the Oracle data server and control all phases of business logic execution. OCI is exposed as a library of standard database access and retrieval functions in the form of a dynamic runtime library that can be linked in by the application.

The Oracle C++ Call Interface (OCCI) is a C++ API that lets you use the object-oriented features, native classes, and methods of the C++ programing language to access the Oracle database. The OCCI interface is modeled on the JDBC interface. OCCI is built on top of OCI and provides the power and performance of OCI using an object-oriented paradigm.

Open database connectivity (ODBC), is a database access API that lets you connect to a database and then prepare and run SQL statements against the database. In conjunction with an ODBC driver, an application can access any data source including data stored in spreadsheets, like Excel.

Oracle offers a variety of data access methods from COM-based programming languages, such as Visual Basic and Active Server Pages. These include Oracle Objects for OLE (OO40) and the Oracle Provider for OLE DB. Oracle also provides .NET data access support through the Oracle Data Provider for .NET. Oracle also support OLE DB .NET and ODBC .NET.

Oracle also provides the Pro* series of precompilers, which allow you to embed SQL and PL/SQL in your C, C++, or COBOL applications.

Overview of Transactions

A transaction is a logical unit of work that comprises one or more SQL statements run by a single user. According to the ANSI/ISO SQL standard, with which Oracle is compatible, a transaction begins with the user's first executable SQL statement. A transaction ends when it is explicitly committed or rolled back by that user.

Note:

Oracle is broadly compatible with the SQL-99 Core specification.

Transactions let users guarantee consistent changes to data, as long as the SQL statements within a transaction are grouped logically. A transaction should consist of all of the necessary parts for one logical unit of work—no more and no less. Data in all referenced tables are in a consistent state before the transaction begins and after it ends. Transactions should consist of only the SQL statements that make one consistent change to the data.

Consider a banking database. When a bank customer transfers money from a savings account to a checking account, the transaction can consist of three separate operations: decrease the savings account, increase the checking account, and record the transaction in the transaction journal.

The transfer of funds (the transaction) includes increasing one account (one SQL statement), decreasing another account (one SQL statement), and recording the transaction in the journal (one SQL statement). All actions should either fail or succeed together; the credit should not be committed without the debit. Other nonrelated actions, such as a new deposit to one account, should not be included in the transfer of funds transaction. Such statements should be in other transactions.

Oracle must guarantee that all three SQL statements are performed to maintain the accounts in proper balance. When something prevents one of the statements in the transaction from running (such as a hardware failure), then the other statements of the transaction must be undone. This is called rolling back. If an error occurs in making any of the updates, then no updates are made.

Commit and Undo Transactions

The changes made by the SQL statements that constitute a transaction can be either committed or rolled back. After a transaction is committed or rolled back, the next transaction begins with the next SQL statement.

To commit a transaction makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions' transactions that start only after the transaction is committed.

To undo a transaction retracts any of the changes resulting from the SQL statements in the transaction. After a transaction is rolled back, the affected data is left unchanged, as if the SQL statements in the transaction were never run.

Savepoints

Savepoints divide a long transaction with many SQL statements into smaller parts. With savepoints, you can arbitrarily mark your work at any point within a long transaction. This gives you the option of later rolling back all work performed from the current point in the transaction to a declared savepoint within the transaction.

Overview of Datatypes

Each column value and constant in a SQL statement has a datatype, which is associated with a specific storage format, constraints, and a valid range of values. When you create a table, you must specify a datatype for each of its columns.

Oracle provides the following built-in datatypes:

Character datatypes

Numeric datatypes

DATE datatype

LOB datatypes

RAW and LONGRAW datatypes

ROWID and UROWID datatypes

New object types can be created from any built-in database types or any previously created object types, object references, and collection types. Metadata for user-defined types is stored in a schema available to SQL, PL/SQL, Java, and other published interfaces.

An object type differs from native SQL datatypes in that it is user-defined, and it specifies both the underlying persistent data (attributes) and the related behaviors (methods). Object types are abstractions of the real-world entities, for example, purchase orders.

Object types and related object-oriented features, such as variable-length arrays and nested tables, provide higher-level ways to organize and access data in the database. Underneath the object layer, data is still stored in columns and tables, but you can work with the data in terms of the real-world entities--customers and purchase orders, for example--that make the data meaningful. Instead of thinking in terms of columns and tables when you query the database, you can simply select a customer.

Overview of Globalization

Oracle databases can be deployed anywhere in the world, and a single instance of an Oracle database can be accessed by users across the globe. Information is presented to each user in the language and format specific to his or her location.

The Globalization Development Kit (GDK) simplifies the development process and reduces the cost of developing internet applications for a multilingual market. GDK lets a single program work with text in any language from anywhere in the world.